The myc oncogene is one of the most common loci of genetic abnormalities in breast cancer, particularly in older women. Paradoxically, its amplification has been associated with rapid proliferation, with malignant progression, and with programmed cell death (apoptosis). Our preliminary studies in vitro with human and mouse mammary epithelial cells and in vivo in transgenic mouse models have suggested that co-overexpression of tgfa and myc genes leads to rapid proliferation, anchorage independent growth, suppression of apoptosis, genetic instability, and malignant progression. The purpose of this grant is to utilize a combination of in vitro and in vivo approaches l) to establish the mechanism of TGFalpha/EGF suppression of Myc-induced apoptosis, 2) to establish the mechanism of Myc disregulation of the cell cycle 3) to determine the consequences of TGFalpha/Myc co-overexpression and the role of Bcl-XL for genetic instability in vitro, and 4) to study the consequences of co-overexpression of Myc with TGFalpha versus Bcl-XL for genetic changes during tumor progression in vivo. Our preliminary data have established a strong influence of TGFalpha/EGF to induce Bcl-XL at the mRNA and protein levels; this may explain anti-apoptotic effects of the EGFR pathway. We have established that Myc strongly activates cdk-2, leading to hyperphosphorylation and inactivation of Rb-1; this may allow G1 shortening in conjunction with cell cycle aberrations when the EGFR/cdk-4 pathway is activated. We have established collaborations to utilize multicolor spectral karyotyping and comparative genomic hybridization (CGH) for in vitro analysis of genetic instability resulting from coexposure of mammary epithelial cells to Myc and TGFalpha or Bcl-XL. An assay of genetic instability due to gene amplification will also be employed. P53 structure and function will be considered as an independent, but interacting variable in Myc-dependent tumor cells. We will use CGH along with a point mutagenesis reporter system for studies in vivo on the relationship between TGFalpha induction of Bcl-XL, apoptosis, genetic changes, and tumor progression. This grant will provide insight into the genetic and biological consequences of Myc oncogene expression, particularly relevant to postmenopausal breast cancer.